I very much doubt it has anything to do with the wire length.
More like with what's at the end of it.

Headphones have a low impedance, usually 32Ω.
The audio line input impedance varies depending on the producer, but it is usually in the range of tens or kΩ (47kΩ and 100kΩ being quite common).

That's ten thousand times more! From the phone's point of view, that may look like an open circuit. Which is just a fancy way of saying "a cable with nothing connected at the other end".

A very easy way to check would be just connecting your headphones at the end of the long cable, perhaps with something like this:



A bit more advanced may be to use a Y splitter that allows to plug in two sets of headphones and instead of the other set just plug in the cable to your audio system. The sound will come out through both the headphones and the audio system but it will prove the concept.